An electrocardiograph is essentially a voltmeter or galvanometer which records changing electrical activity in the heart by means of positive and negative electrodes. Electrocardiography is the process of recording the potential differences at these electrodes. An electrocardiogram (hereinafter referred to as an ECG) is a representation on paper, or an oscilloscope display, or a computer screen, of electrical activity in the heart. An ECG is basically obtained by monitoring the voltage changes at electrodes connected to test leads, which leads are, in turn, connected to the electrocardiograph. The electrodes of the test leads can be connected to various regions of the body. In humans, the electrodes are often placed on the chest (precordial chest ECG), while in animals, the electrodes are typically placed on the limbs (body surface limb ECG). Sometimes, electrodes are placed inside the cardiac chambers.
ECGs are used by veterinarians and physicians to diagnose cardiovascular diseases. They also provide a valuable tool for biomedical research, including but not limited to preclinical drug discovery and toxicology studies. When pharmaceutical companies submit new drug applications to regulatory agencies, such as the United States Food and Drug Administration, such agencies often require information about cardiovascular function relative to the introduction of the drug or its metabolites. Drug-induced cardiovascular anomalies revealed in the human population subsequent to the introduction of a new pharmaceutical product can result in the termination of such a product (i.e., the drug will be prohibited by the regulative agency from use in the manner proposed in the application). There is, therefore, a pressing need to develop models for use in drug development that can predict cardiovascular problems in humans prior to use in actual human subjects. Adverse indications involving the heart would ideally be discovered early in the process of discovery, before large research investments had been made in developing the drug candidate.
Laboratory rats and mice are typically the first animals to be exposed to potential drug candidates. Although ECGs are recorded in rats and mice, for several reasons, the rat and mouse are not considered the ideal animal model for cardiovascular screening. With regard to obtaining ECGs from laboratory rats and mice, the following three approaches are typically used: (1) immobilization, (2) telemetry or (3) recording platform. Each of these approaches have shortcomings as is described in greater detail herein.
Immobilization involves connection of electrical leads (e.g. coated copper wire) to the limbs of the rat. The rat must be immobilized during such studies because movement can create signal artifacts attributable to the electrical activity of other muscles. To establish a reliable connection and ensure that the rat's fur does not interfere with the connection, it is necessary to pierce the skin of the rat. The piercing of the rat's skin can be accomplished by the use of hypodermic needles or alligator clips. The rat is often anesthetized or at least restrained during these studies to reduce signal noise due to movement and because the piercing of the rat's skin can be painful.
The immobilization technique, thus, results in several shortcomings. The ECGs obtained are those of an immobilized animal that may reflect restraint stress in those ECGs as well as other physiological indicators. If anesthetic is used to accomplish the restraint, the ECGs represent a cardiovascular system under the influence of another variable and may not accurately portray the effect of the drug on the cardiovascular system. Both the immobilization and the presence of anesthesia affect heart rate and therefore can affect the resulting ECG. The anesthetized animal may behave differently according to the uptake and/or clearance of the drug, because drug uptake by tissues and clearance from tissues is often through the blood stream and the rate of blood flow is governed by the heart rate. Drug-induced changes in electrocardiograms may be associated with delayed metabolism and/or clearance of a drug, as in the case of terfenadine, a drug strongly associated with the cardiovascular anomaly known as QT interval prolongation. If an anesthetic is used instead of a restraint, there still could be an effect on body temperature because certain anesthetics can lower body temperature. Body temperature is another variable that affects drug uptake, clearance, and general metabolism. Larger animals require larger and sturdier restraints to effectively prevent movement. In many breeds of dog, it is sufficient for a practitioner to hold the dog on an examining table. With other large animals, a strong restraint is necessary. It is therefore desired to provide a device and method for electrocardiography that can be used on animals of all sizes (mouse, rat, dog, monkey, and even a human) that at least does not require that the animal be anesthetized, and, in the case of rodents or other small animals, does not require that the animal be restrained.
The telemetry method of cardiovascular screening in rats requires that deep body surgery be conducted to open the body cavity for implantation of a small, battery-powered and sterile transmitter into the rat. To detect electrical activity in the heart, leads from the transmitter are attached to the rat's heart or blood vessels adjacent to the heart. Before using the rat with such an implant for research and testing, the rat requires several days of recovery after the surgery. After such recovery, the rat can be allowed to move without restraint within a specialized cage and tests, such as an ECG, can be taken on the freely moving rat.
The telemetry device and method for electrocardiography has several shortcomings. The battery in the transmitter has a finite lifetime, thereby limiting the amount of time and number of ECGs that can be obtained from the subject rat. The rat is exposed to a magnetic field that induces the battery to turn on and induces the transmitter to send signals to a receiving antenna. The battery is turned off by a subsequent pulse of the magnetic field. By judicious use of these fields, the battery lifetime is somewhat conserved, but the scope of the research is necessarily constrained by the lifetime of the battery. The telemetry approach is also very sensitive to magnetic fields that are not part of the equipment, which cause problems with the operation of the battery. Likewise, the telemetry approach is sensitive to transmitter/receiver systems from adjacent animal studies, which interfere with the collection of the signals from the animal. Thus, animals with these implants need to be kept close to the antennas to ensure that the information is properly collected and need to be isolated from one another to ensure that the transmission of the electrocardiography signals do not interfere with one another. The need for proximity to the receiving antenna, the sensitivity of the antenna to stray radiofrequency interference and the susceptibility of the battery to other magnetic fields, all contribute to a burden on the research facility. Moreover, the need for deep body surgery to implant the transmitter and the subsequent recovery time place a strain on the animal and increase the risk of infection. Therefore, it is desired to provide a device and method for electrocardiography that does not require intensive, deep body surgery and is not constrained by the manner of limitations imposed by the use of a battery and antenna.
The use of a recording platform for obtaining ECGs in mice is a relatively new approach. This approach does not require that the mouse be restrained, as in the immobilization method, and does not require surgery to implant a device, as in the telemetry method. The recording platform is permeated with electrodes. When a mouse is placed on the platform and has all of its paws in direct contact with these electrodes, a signal is obtained that can be viewed on a computer screen and recorded. The platform is typically elevated and the mouse must be monitored by a technician who triggers the recording of the ECG once the technician detects a usable signal on the computer screen. These platforms are manufactured under the name AnonyMOUSE™ by Mouse Specifics, Inc. The recording platform has the advantage of being non-invasive (no surgery required) and is not painful to the animal since no leads must be attached to the skin. However, the use of a recording platform has several shortcomings.
The primary shortcoming is that the recording platform requires the animal to remain still and in contact with the leads for acquisition of a signal to produce an ECG. A rearing animal, for example, would not produce a useable signal. Further, because the animal is not contained, there is a risk of escape or injury to the animal if it leaps from the elevated platform. If the animal was contained within cage walls to prevent this risk, it would be able to rear and lean on the cage walls, resulting in a loss of signal and intermittent electrocardiograph readings. Finally, the animal must be handled in order to be transferred from its home cage to the recording platform, and some amount of time would be required to allow the animal to return to resting status after the stimulus of handling. Accordingly, it is desired to provide a device and method for electrocardiography on freely moving animals that does not require handling of the animal, does allow for containment of the animal, is not affected by normal behavior such as rearing, and is not limited by intermittent electrocardiograph readings (as results from the required contact of the animal's paws to the elevated platform on the recording platform technique). It is also desired that the acquisition of recordings be independent of human intervention rather than requiring monitoring by a technician as in this recording platform technique.
A few other points specific to electrocardiography in animals are worthy of note. Surface ECGs are normally obtained by connecting an electrically conductive test lead to the skin, often in combination with an electrically-conductive gel to improve electrical contact between the wires in the lead and the skin. This technique must be modified for animals that have a thick fur coat since the hairs of the fur coat can prevent complete contact between the test lead and the skin. Shaving of the fur may be insufficient to make these connections as the fur cannot always be shaved close enough to completely remove the hair without also damaging the skin. For humans, to get good contact, electrically conductive gels are used to make contact with the skin. Animals, such as most breeds of dogs, can be monitored using devices which pinch deep into the skin, such as alligator clips, because they are tolerant of the discomfort of using this method. Other animals, such as rats, are intolerant of such methods and must be anesthetized or restrained in tubes or other devices which prevent them from moving in response to the discomfort of the measurement technique. During even short periods of restraint, such animals are stressed significantly, resulting in changes to their heart rate, blood pressure, circulating concentrations of stress hormones, and metabolic parameters. Neither a restrained animal, nor an anesthetized animal, is representative of normal physiology. It is therefore desired to provide a device and method for electrocardiography that can be used on various types of animals without restraint, the use of anesthetic, the requirement to shave the animal, or the use of conductive gels.